Trees And Shrubs

Growth and Age of Trees

The rate of growth of a tree during its lifetime can be charted on a graph. At first, the curve rises until the tree reaches the age when it attains its greatest growth, then it slowly, but continually, falls until it has almost reached the zero level. After this the tree does not grow in height but may thicken in diameter and attain an age of several hundred years. This growth curve is generally the same for all trees, varying only in rate and point of greatest growth, according to the individual species.

Trees are divided roughly into three groups according to their rate of growth —- fast, moderate and slow-growing. The first group is distinguished by fast growth in the first years with culmination point at an early age, after which the rate markedly decreases. The tree usually has a short life. Fast-growing trees, therefore, do not attain great heights; on the contrary, these are attained by the moderate and slow-growing species. In slow-growing trees the rate of growth is slower in youth, extends over a greater period of time, and reaches its peak at a more advanced age; the downward curve is more protracted and the tree has a longer life span (fir, spruce, beech).

The tallest and largest trees are the sequoias of North America (Sequoiadendron giganleum Buchh. And Sequoia sempervirens EndlJ, the latter attaining heights of up to 112 metres or more. Heights of 80 to 90 metres are reached by some other west American conifers, e.g. the giant fir, noble fir, Douglas fir and sugar pine. Of the European trees the tallest is the European silver fir with a height of over 62 metres and the Norway spruce, which also attains a height of over 60 metres.

The annual increase in height is fairly rapid in most trees, occurring within the brief period of four to five weeks. Growth begins at the same time as the tree comes into leaf. For the first few days it is slow, then comes a period of rapid growth — one to two centimetres a day — followed by one to two weeks of slower growth, and ending with the appearance of a terminal bud at the tip of the shoot. This way of growth is characteristic of the Scots pine, spruce, fir, ash, beech and many others, the period usually terminating at the end of June. There is, however, another group of trees, whose annual growth is spread out over the whole period of vegetation and lasts from three to four months. Included in this group are the poplar, alder, birch, cypress, larch and dawn redwood. Since the annual growth begins early in spring, usually before the tree’s assimilation organs are fully developed, the substances required are taken from the previous year’s store; the extent of growth is thus mainly influenced by the weather of the preceding year.

As regards growth in diameter the case is somewhat different. In all woody plants it takes place throughout the period of vegetation, from the time the tree comes into leaf until the leaves fall in the autumn, and the extent is influenced by the weather of the current year. In some trees it equals two to three centimetres in a favourable “year. Of the European trees great thickness is attained by the plane, chestnut, oak, lime and sycamore, and of the conifers by the fir and to a somewhat lesser degree by the spruce. The “big trees” (Sequoiadendron) of California attain diameters of more than ten metres.

The attainable thickness of the trunk goes naturally hand in hand with the tree’s attainable age. There are great differences amongst the individual species. On the one hand, there are the short-lived trees, and on the other, the long-lived ones that live up to ten times longer. The first group includes trees with a more rapid life cycle, which grow fast when they are young, bearing fruit at an early age and ageing soon. This, however, does not mean that ihey have a short life span compared with that of man. The aspen, goat willow, birch and mountain ash may attain 100—150 years. Twice that age, from 200 to 300 years, is reached by the eastern cottonwood, hornbeam, alder, pine and larch; about 500 years by the beech and sycamore; and 700 years by the spruce and fir. The oldest trees in central and western Europe are the lime, yew and oak, which may live for more than a thousand years. An even greater number of years is attained in the Mediterranean region by the oriental plane, chestnut and the cedar of Lebanon — as many as two thousand; and on the American continent there are trees that are much older. Up to 2500 annual rings were counted on the stump of a redwood and some giant sequoias are 3500 to 3800 years old. About fifteen years ago a stand of bristle-cone pine (Pintts arislata Engelm.) was discovered in the Rocky Mountains of Nevada, in which the oldest trees had 4200 annual rings.

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